Semiconductor laser diodes of the type mentioned above have become important components in the technology of optical communication, particularly because such laser diodes can be used for fibre pumping and other high power laser diode applications. They allow the design and development of all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability of such systems. Other uses of such high power laser diodes include cable TV (CATV) amplifiers, printing applications, and even medical applications. However, the invention is in no way limited to BASE laser diodes, but applicable to any semiconductor laser diode of comparable design.
A typical semiconductor laser diode, e.g., an AlGaAs ridge waveguide laser diode consists of a (strained) quantum well active region sandwiched by two AlGaAs cladding layers. The first cladding layer, which is grown first onto the substrate, is commonly referred to as the lower cladding layer, and is typically n-type doped. The second cladding layer, which is grown second onto the substrate, after growth of the active region, is commonly referred to as the upper cladding layer and is typically p-type doped. The entire semiconductor epitaxial structure is grown on a GaAs substrate. A first electrode metallization provides electrical contact to the first cladding layer and a second electrode metallization provides electrical contact to the second cladding layer. Typically the first electrode covers the opposite surface of the wafer from that on which the epitaxial layers are grown, and the second electrode covers at least part of the ridge waveguide. However, other doping arrangements and locations of electrodes are also possible.
Generally, such a semiconductor laser diode can be operated in two different modes. Firstly, the device can be soldered with the first electrode onto a carrier or submount, which is referred to as a junction-side-up mounted laser diode. Typically, narrow-stripe (single-mode) lasers with a ridge width of a couple of microns are soldered in this way. Secondly, the device can be turned upside down and soldered with the second electrode onto a carrier or submount, which is referred to as a junction-side-down mounted laser diode. Typically broad area (multi-mode) lasers, BASE, with a ridge width of the order of 100-200 μm are soldered in that way. It should be noted that this invention may be preferably applied to junction-side-down mounted BASE laser diodes. However, it should be clear that the invention is in no way limited to such devices. In particular, the invention is not limited to ridge waveguide lasers as described above, but applicable to other designs of semiconductor laser diode, for example such as a buried heterostructure laser diode.
One of the major problems of all semiconductor laser diodes is the degradation in the end section area, particularly in the vicinity of the laser diode's front facet. This degradation is believed to be caused by uncontrolled temperature increase in the facet regions (or end sections) of the ridge waveguide, especially at high power outputs. The temperature increase may be caused by unwanted carrier recombination in these regions and heating due to free carrier injection.
The local current in the end section of the laser diode's ridge waveguide, and other parts of the laser diode, is generated by the injection current driving the laser diode. Thus, to reduce the local current density and to finally prevent current flow within the laser diode's end sections, and thus the unwanted carrier recombination, it is known to reduce the current injected into these end sections. Various designs for reducing the current injected into the end sections have been tested and described, including contact lift-off, removing the contact by etching, or otherwise interrupting the contact around these regions. Some of the tested and realized designs failed due to material, processing, or reliability problems, some show undesirable side effects, and some are just impractical or too difficult to implement.
Some known ways to prevent the above described carrier recombination in the laser diode's facet regions shall be described in the following.
One attempt is disclosed in Itaya et al. U.S. Pat. No. 5,343,486. It shows a compound semiconductor laser diode with a current blocking region formed in one facet portion of the laser diode structure. Thus, Itaya discloses a solution for reducing the current injection. The transition between the pumped and the unpumped sections is not addressed.
Yu et al. U.S. Pat. No. 6,373,875 discloses a plurality of current-blocking layers for a ridge waveguide laser diode, one each over each of the end sections of the laser diode's ridge waveguide and two separate blocking layers fully covering the remaining body right and left of the ridge waveguide. This structure thus has several layers which are not laterally contiguous and the interruption just at the edge of the waveguide may lead to undesired effects. The transition between the pumped and the unpumped sections is not addressed. Also, it appears questionable whether this approach is viable for BASE laser diodes.
Sagawa et al. U.S. Pat. No. 5,844,931 discloses a “windowed” current-blocking layer for a ridge waveguide laser diode, the blocking layer covering the ridge and the whole body with a longitudinal opening, i.e. a window, over the centre part of the ridge. Sagawa teaches an electrode that extends across an abrupt edge to the insulation layer, but the transition between the pumped and the unpumped sections is not specifically addressed.
Yariv U.S. Pat. No. 4,791,646 discloses a tailored gain broad area laser made by a “halftone” process whereby patterns of dots (used for example in newspapers to reproduce the impression of shading, using only black and white areas), are employed to produce non-uniform two-dimensional current injection and corresponding spatial gain. The preferred embodiment is a laterally asymmetric device which produces a narrow single-lobed optical far-field pattern. The two particular problems that Yariv is seeking to overcome are filamentation (an unstable preferentially driven section of the waveguide) and multimode operation, leading to broad, unstable, optical far-fields, that occur in wide waveguide devices that have a substantially uniform lateral spatial gain profile. In the sole embodiment, Yariv teaches non-uniform lateral gain distribution to break the left-right inversion symmetry of the laser to provide improved discrimination of the fundamental mode over the higher order modes, for the purposes of improved mode control. For higher powers, Yariv teaches that the device should be more laterally asymmetric. This required asymmetry is a burden that the present invention does not need, and a symmetrical BASE laser is easier to manufacture and to control. Also, Yariv teaches a gradation of current injection across the whole area of a device, with no regions having full current injection. This is certainly a limitation with regard to the maximum light power output and thus not desirable for high power lasers. In contrast to that the present invention concerns only a gradation in current injection at the boundaries between areas of full and zero current injection.
A different, but rather successful approach for a ridge waveguide laser diode is an “isolation layer” process to achieve the desired unpumped end sections. This approach differs from earlier ones in the way that an additional thin isolation layer is placed between the semiconductor contact layer and the metal contact at the laser diode end sections. The semiconductor contact layer may even be partly removed or truncated. Such a design is disclosed by Schmidt et al. U.S. Pat. No. 6,782,024, assigned to the assignee of the present invention and incorporated herein by reference, showing a solution with unpumped end sections by providing an isolation layer as a current blocking layer of predetermined position, size, and shape between the laser diode's semiconductor material and the metallization. However, the solution disclosed by Schmidt et al. has an abrupt transition between the area where the electrode contacts the laser diode, and where it does not. This transition between the pumped and the unpumped sections is not specifically addressed.
Another successful approach for a semiconductor laser diode, in particular a BASE laser diode, is disclosed in the co-filed UK patent application GB0513039.8 (N. Matuschek et al., “High Power Semiconductor Laser Diode”), also assigned to the assignee of the present invention. The solution described therein is characterized by singular carrier injection points (or a tape-like conductor), preferably wired contacts, extending closely spaced essentially along the ridge waveguide, appropriately arranged for providing one or two unpumped end section(s) with reduced, preferably minimized, injection of carriers and consequently reduced, preferably minimized, current density at the end sections, thereby also providing a spike-less transition between pumped and unpumped sections.
Whereas, as shown and discussed above, unpumped end sections provide often successful solutions to block current flow in one or both end sections of a high power laser diode and thus prevent overheating and resulting catastrophic optical mirror damage (COMD) breakdowns, there are still occasions where this does not suffice. In addition, most of the above-described solutions concern junction-side-up mounted ridge waveguide lasers and do not take into account the structural differences of other laser designs, e.g. junction-side-down mounted broad area single emitter (BASE) laser diodes. In other words, the above-described solutions are not necessarily applicable for such other to designs as BASE laser diodes. To focus again, one of the main issues is the abrupt transition between regions of zero and full current injection (or pumped and unpumped sections) on the top, i.e. non-substrate, surface of the laser diode which causes undesired current spikes as described below.
Simulations of the current distribution in high power laser diodes show a strong peak in the current density, i.e. a current spike, at the transition between the pumped and the unpumped section of the laser diode. This current spike probably leads to a local stress of the material in the region concerned. Material degradation in this region of laser diodes which have been operated for some time can be observed and is visible in electric-beam-induced current (EBIC) signatures of the material at this region of the device. This effect is especially prominent at very high powers, with high current densities.
The present invention aims to provide an improved current blocking layer or structure that provides an improved current distribution in the vicinity of the laser diode's end sections. The improved current blocking layer may provide a powerful stable light output under all operating conditions, and avoid the above-mentioned end section degradation. Another aim is to provide an economical manufacturing method, allowing reliable mass production of such high power laser diodes.
A further aim is to reduce the complexity of the laser diode structure and to keep the number of additional structural components of the laser diode at a minimum.